1. Field of the Invention
The present invention relates, in general, to a rigid flexible printed circuit board (PCB) and a method of fabricating the same and, more particularly, to a rigid flexible PCB and a method of fabricating the same, in which a polyimide copper clad laminate is not used during the fabrication of the rigid flexible PCB so as to avoid an increase in cost resulting from use of the polyimide copper clad laminate and poor reliability of adhesion at an interface between different materials.
2. Description of the Related Art
In accordance with the recent trend toward miniaturized, light, and slim electronic devices, light, slim, and high density PCBs are in demand. Accordingly, the use of a multilayered flexible PCB capable of satisfying the above demand is rapidly growing. However, a process of fabricating the multilayered flexible PCB is complicated and has many stages compared to a process of fabricating a general rigid PCB, thus costs and defective fractions are high. Of multilayered rigid PCBs, recently, a rigid flexible PCB having a flexible portion has been employed to produce folding-type mobile phones, thus it is expected that demand for rigid flexible PCBs will rapidly grow. The flexible portion of the rigid flexible PCB reduces workability in a process of producing goods and increases defective fractions. The development of special material and a process employing same contribute to solving the above problems, thereby assuring superiority over rival companies. In order to fabricate the flexible PCB, the formation of microcircuits using thin and strong material must be feasible. In other words, the selection of material and process is important. Polyimide resin has mainly been used as the material because there is no material usable as a substitute for polyimide resin with regard to heat resistance, mechanical strength, fire retardancy, and electrical properties. However, currently, material capable of being used as a substitute for polyimide resin has been developed with regard to high frequency response, moisture resistance, dimensional stability, and cost.
In accordance with miniaturization and integration of electronic parts, various multilayered circuit boards capable of having the electronic parts mounted thereon have been developed. Of them, many studies have been made of a rigid flexible multilayered PCB (hereinafter, referred to as “rigid flexible PCB”) which is capable of minimizing the volume occupied by a substrate and of being modified in three dimensions.
In the rigid flexible PCB, it is possible to combine a plurality of boards in one PCB without additional connectors or cables for connecting boards to each other, thus there are advantages in that there is little delay or distortion of an electric signal and a volume of space for mounting can be reduced. Accordingly, it is considered that it can be used for a static & dynamic application which allows bending only once and an application for a few hundred thousand cycles. However, since costly flexible polyimide copper clad laminate is employed, the fabrication cost is 3-4 times as high as that of a typical rigid flexible PCB. As well, poor adhesion strength at an interface of the prepreg (or bonding sheet), which is used as an interlayer adhesive to form multilayered rigid portions, and polyimide has been indicated as a disadvantage in the fabrication of the PCB and in the realization of reliability for a long time. Particularly, recently, the cost burden of polyimide material is growing heavier in accordance with the decrease of the price of PCBs, and the use of connectors or cables, which belongs to a con to con connection manner, is increasing again in some fields. However, in consideration of delay of the electric signal or the trend toward light, slim, short, and small electronic products, the fabrication of a low-priced rigid flexible PCB employing a novel process is in demand.
FIGS. 1A to 1I are sectional views illustrating the fabrication of a rigid flexible PCB, according to conventional technology.
As shown in FIG. 1A, a prepreg 101 through which a window (A) is formed at a predetermined position thereof is prepared.
As shown in FIG. 1B, polyimide copper clad laminates which include polyimide films 102a, 102b and copper foil layers 103a, 103b are layered on both sides of the prepreg 101.
As shown in FIG. 1C, dry films 104a, 104b or liquid photoresists are applied on the copper foil layers 103a, 103b. 
As shown in FIG. 1D, artwork films having predetermined patterns printed thereon are attached to the dry films 104a, 104b, exposed, and developed to form predetermined etching resist patterns. Furthermore, the resulting structure is dipped in an etching solution, and, at this time, the dry films 104a, 104b act as an etching resist. Thereby, portions of the copper foil layers 103a, 103b, which correspond in position to portions other than the predetermined patterns of the dry films 104a, 104b, are removed.
As shown in FIG. 1E, the dry films 104a, 104b are removed using a stripping solution to form predetermined circuit patterns 103a, 103b. 
As shown in FIG. 1F, coverlays 105a, 105b are applied on the circuit patterns 103a, 103b which correspond in position to the window (A) at the predetermined position, and are then pressed.
As shown in FIG. 1G, prepregs 106a, 106b which are prepared in FIG. 1a, are layered on the circuit patterns 103a, 103b so as to expose a portion of the coverlays 105a, 105b. 
As shown in FIG. 1H, polyimide copper clad laminates which include polyimide films 107a, 107b and copper foil layers 108a, 108b are layered on the prepregs 106a, 106b which constitute outermost layers, and are then subjected to a photolithography process to form predetermined circuit patterns 108a, 108b. 
As shown in FIG. 1I, the step of FIG. 1H is repeated once to apply coverlays 113a, 113b on portions of the outermost circuit patterns 108a, 108b, which correspond in position to the window (A) at the predetermined position, and the resulting structure is pressed to create the rigid flexible PCB.
As a related technology, U.S. Pat. No. 6,745,463 discloses a method of fabricating a rigid flexible PCB.
However, in the conventional PCB, a polyimide copper clad laminate and a prepreg made of different materials constitute a rigid region, thus adhesion strength is reduced. To avoid this, the polyimide copper clad laminate must be surface treated using additional plasma (or corona) treatment or mechanical brushing.
Additionally, in the conventional PCB, since a coverlay is laminated on the polyimide copper clad laminate on which a circuit pattern is formed, the circuit pattern is curled toward a curved portion of the coverlay due to coefficient of thermal expansion (CTE) of adhesive of the coverlay when thermal hysteresis occurs (surface mount process, etc.), thus mounting of surface mount devices is unsatisfactorily conducted, resulting in increased defective fractions.
Another problem is that, since cost of the polyimide copper clad laminate is higher than that of the prepreg, it is impossible to form a low-priced rigid flexible copper clad laminate.